EPIC-Oxford and Adventist Health Study-2 (AHS-2) are the two ongoing studies of diet and disease that contain a large number of vegans. This past week, AHS-2 released a report on colorectal cancer rates among various diet groups (1).

In summarizing the research on diet and colorectal cancer to date, the researchers write:

“Among dietary factors thought to influence risk, the evidence that red meat, especially processed meat, consumption is linked to increased risk and that foods containing dietary fiber are linked to decreased risk has been judged to be convincing. The evidence for a link to decreased risk has been judged as probable for garlic, milk, and calcium. Evidence for other dietary components is considered limited.”

Because of the link with red and processed meat, nutritionists had expected that vegans and vegetarians would have a lower risk for colorectal cancer. To date, EPIC-Oxford has not shown a lower risk for vegetarians (results can be seen in Cancer, Vegetarianism, and Diet).

Better news comes from the latest AHS-2 report. After an average follow-up of 7.3 years, the lacto-ovo vegetarians and vegans had a lower risk of colorectal cancer (18% and 16% respectively), but the findings were not statistically significant. Here are the risk ratios and confidence intervals for each group:

The only group with a statistically significant lower risk was the pesco-vegetarians (who eat no meat other than fish), with a 43% lower risk that was highly statistically significant. Despite this, the difference between the pesco-vegetarians and vegans was not statistically significant, though close (.68, .43-1.08).

When lumping all the groups together (semi-, pesco-, lacto-ovo, and vegan) and comparing to non-vegetarians, the “vegetarians” had a statistically significant, 22% lower risk of colorectal cancer (.78, .64-.95).

Results were adjusted for age, race, gender, education, exercise, smoking, alcohol, family history, peptic ulcers, inflammatory bowel disease, diabetes mellitus, aspirin, statins, prior colonoscopy or flexible sigmoidoscopy, supplemental calcium, supplemental vitamin D, calories, and hormone therapy. Subjects were excluded if they had a prior diagnosis of cancer (except for non-melanoma skin cancer).

Overall, colorectal cancer rates in AHS-2 were lower than the population at large and AHS-2 non-vegetarian eat very little meat (54.5 g/d or just 4.5 servings per week).

Could a higher calcium intake for vegans lower their risk of colorectal cancer? In AHS-2, the vegans had slightly lower calcium intakes (801 mg/day) than the pesco-vegetarians (913 mg/day) and non-vegetarians (882 mg/day). But in EPIC-Oxford, where the results were not as positive for vegetarians, vegans had much lower calcium intakes (583 mg/day compared to 1,005 mg/day for non-vegetarians) (2).

Another difference between the EPIC-Oxford and AHS-2 groups is that the AHS-2 vegans had a substantially greater intake of both dietary fiber and vitamin C.

Does eating fish protect against colorectal cancer? The researchers write, “The existing literature provides some, although inconsistent, support for a possible protective association for fish consumption, particularly for rectal cancer; evidence for omega-3 fatty acid consumption is limited and inconsistent.” In other words, it’s not clear.

In summary, colorectal cancer rates in vegans in AHS-2 are promising, but it might be important for vegans to follow calcium recommendations not only for bone health but also to prevent colorectal cancer.

References

1. Orlich MJ, Singh PN, Sabaté J, Fan J, Sveen L, Bennett H, Knutsen SF, Beeson WL, Jaceldo-Siegl K, Butler TL, Herring RP, Fraser GE. Vegetarian Dietary Patterns and the Risk of Colorectal Cancers. JAMA Intern Med. 2015 Mar 9. [Epub ahead of print] | link

2. Average calcium intakes for EPIC-Oxford calculated from Nutrient Intakes of Vegetarians and Vegans.

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